High-temperature applications based on a purely physical drying principle usually require baking of the coating film at elevated temperatures, in order to obtain the necessary chemical and physicochemical stability. This is particularly disadvantageous, since the limitation on oven size means that not all materials can be subjected to forced drying. Furthermore, as the size of the object increases, it becomes increasingly difficult to achieve an object temperature of 150-250° C. as required for the baking operation.
In the prior art, silicone resin compositions are known which comprise substantially methoxy-functional methylsilicone resins. They are cured using, for example, titanium tetrabutoxide. One such process is described in EP 0802 236 B1, for example.
Alkoxy-functional methyl silicone resins such as SILRES® MSE 100, for example, have already been used here for many years, in combination with various curing catalysts, for coatings which cure at room temperature and possess high-temperature stability for temperature ranges up to 650° C.
In spite of the rapid cure rate of the methoxy-functional methylpolysiloxane resins, and the heat stability, they also have certain performance disadvantages for the paint formulator. For instance, the methylsilicone resins exhibit poor compatibility with other organic compounds, and, moreover, are very hard and brittle, as manifested particularly on thermal exposure of the cured coating.
For this reason, the majority of coatings for high-temperature application are made flexible by means of suitable, platelet-shaped fillers, such as mica, for example, and inorganic temperature-stable color pigments, to allow the sharp change in volume of the substrate on heating and cooling to be compensated. Furthermore, some methyl-silicone resins also exhibit an adverse effect on the coating system: for example, as a result of the PDMS character (PDMS=polydimethylsiloxane) of the methylsiloxanes, there may be surface defects, such as that known as cratering, for example.
An example of another manifestation of the incompatibility is that certain catalysts are completely unsuitable for use as crosslinking catalysts for alkoxy-functional methylsilicone resins, since the poor miscibility with the binder means that curing does not occur. For this reason, titanates, such as tetra-n-butyl titanate, for example, tend to be used for methylsilicone resins.